1. Field of the Invention
The present invention relates to rubber compositions for tire treads, which are suitable not only for use on ordinary paved roads, but also particularly suitable for use on icy and snow-covered roads.
2. Description of the Prior Art
Conventionally, for safe travel on icy and snow-covered roads, spike tires have been used in many cases. However, spike tires generate dust as well as noise pollution. In view of these environmental problems, the use of spike tires has become questionable.
Consequently, in recent years, a tire without spike pins, the so-called studless tire, was introduced. However, at its present state of development, this studless tire is inferior to the spike tire in three areas: grade climbing ability, braking performance, and standing start performance. Also, the studless tire usually uses a large amount of softening agent or plasticizer because of the necessity to lower the hardness of the tread portion. In most cases this softening agent or plasticizer migrates into other portions of the tire, such as the belt and side wall portion, which are located contiguous to the tread rubber, or it is lost from the tread rubber by vaporization, etc., thereby causing degradation of the original performance of the tread rubbers.